LuxBlend light groups
After fiddling with this issue, I found out this is a bug. I uninstalled Luxrender, removed the add-on from Blender, restarted my machine and installed a different version. Using Luxrender 1.6 test version No OpenCL, I have the Light Groups available in Render Layers panel. The Materials Panel preview is still black. I couldn't find a Group Node (maybe just because I'm unfamiliar with Lux), but if I don't use Nodes, I can create Light Groups in Render Layers panel and assign light materials to the Light Groups.